1. Field of the Invention
The present invention relates to the field of radio frequency communications and radio frequency technologies, more specifically the cancellation of the cross-polarized components of radio frequency transmissions. In particular, the invention relates to methods of forming adaptive cancellation schemes to decrease cross-polarized components using the co-polarized components from the orthogonal ports of an antenna array.
2. Description of Related Art
In radio frequency (RF) communications, radio wave polarization is the direction in which radio waves vibrate when propagating. There are two basic types of radio frequency polarization: linear polarization and circular polarization. Linearly polarized signals can vibrate in only one direction, either horizontally or vertically. On the other hand, circularly polarized signals propagate in a circular fashion with either left handed or right-handed rotation, forming a helical shape.
In most cases, an antenna feed can be regarded as two ports with orthogonal polarizations; for example, horizontal polarization (HP) and vertical polarization (VP). Each of the ports receives signal components that have the same polarization called co-pol. However, due to the imperfections of the polarizations of the receiver, each of the ports receives a small portion of orthogonal signal component called cross-pol which introduces interference to the signal. Thus, the received signal of HP and VP can be further broken down into four different parts, namely HP co-pol, HP cross-pol, VP co-pol and VP cross pol.
Since the cross-pol component is relatively small compared with the co-pol component, their influences are often ignored. For example, in the application of smart antenna arrays, digital beam forming techniques (DBF) may be used to increase antenna gain. These methods involve applying proper weightings to each element component in order to maximize co-pol in-phase alignments, resulting in increased signal strength in desired directions.
Although the cross-pol is small, it introduces interference and may limit system performance. In many applications, the cross-component should be attenuated below certain levels so that the antenna may function properly, either to fulfill performance parameters or to reduce interference to a usable level. One possible solution to this issue is using DBF, but considering both the co-pol and cross-pol component as optimization criteria. However, due to the limitation of degrees of freedom, the resulting additional constraint points may compromise average performance. Any improvement in cross-pol mitigation performance directly results in a degradation of co-pol performance. As a result, system performance cannot be improved by just adding extra constraint conditions; instead, additional auxiliary information will be necessary.
Our proposed invention takes advantage of the relationship between the four components of received signals, HP co-pol, HP cross-pol, VP co-pol and VP cross-pol, to minimize the cross-pol in each polarization. Since the degrees of freedom of the system have been greatly increased, the cross-pol component can be reduced significantly without the scarification of the co-pol performance, thus improving the performance of the whole system.
The following references are presented for further background information:
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